This invention relates to fire detector systems including ionization smoke detectors capable of assuming different electrical states, which are connected by signal lines to control and indicating means.
Fire detector systems are finding increasing use for the protection of human life and of valuables; they include fire detectors located with objects to be protected, and control and indicating equipment connected to the detectors via communication means. Distinguished among fire detectors are ionization smoke detectors, as these are capable of detecting a fire at a sufficiently early stage to allow timely deployment of suitable fire-fighting means, especially for the protection of human life. Such systems may be designated as early-warning systems.
The operation of ionization smoke detectors is based on the presence of smoke or aerosols affecting the ionic current flowing in an ionization chamber. In such a detector, ambient air entering a measurement chamber is ionized by a weakly radioactive source, thereby permitting an ionic current to flow between electrodes. If smoke or, more generally speaking, a combustion aerosol enters the measurement chamber, atmospheric ions are adsorbed onto the aerosol particles, thereby significantly reducing their mobility. As a result, the ionic current is decreased. If the change in current exceeds a certain critical value, an alarm signal is produced and transmitted to control and indicating means.
With fire detector systems there is a concern with false alarms. In the case of ionization smoke detectors, there is particular concern with the sensitivity of detectors to fast air currents, to condensation, or to shielding of the radioactive source by dust or corrosion, as these phenomena have an influence on the ionic current similar to the presence of combustion aerosols. Since the attendant change of the ionic current results in increased detector sensitivity, the tendency to false alarms is increased also. False alarms are of particular concern when an alarm signal causes the activation of automatic fire extinguishing means or the mobilization of external fire-fighting forces.
For the prevention of false alarms due to fast air currents, suitable designs have been developed for the air inlet openings, e.g., in accordance with published German Patent Document DE-B2-2415479. To prevent malfunctioning of ionization smoke detectors due to condensation the electrodes may be heated. As disclosed in published German Patent Document DE-B2-2537598, this may involve the use of heat as inevitably produced by the electronic circuitry.
Published European Patent Document EP-A1-0070449 discloses evaluation of measurement data after their transmission to control and indicating means. This involves forming a quiescent value based on individual detector measurements, and storing the quiescent value in a quiescent-value memory. Based on a measured actual value and a comparison value stored in a comparison memory, an updated comparison value is computed and stored in the comparison memory. Depending upon the outcome of a comparison between the updated comparison value and a threshold value, either (i) indicating equipment is activated or else (ii) an updated quiescent value is formed based on the measured actual value and the previously stored quiescent value, and the updated quiescent value is stored in the quiescent-value memory. In this fashion it is possible to compensate for slow changes in the detector, e.g., due to soiling, and to maintain stable detector sensitivity over extended time periods.
Published German Patent Document DE-A1-2428325 discloses the choice of a condensation-inhibiting chemical composition for the plate separating the measurement chamber from the reference chamber, to prevent condensation and attendant deterioration of the electrical insulation of the ionization measurement chamber.
For the prevention of false alarms due to shielding of the radioactive source by soiling, published Japanese Patent Document JP-PA-47-93018 discloses proportioning of the leakage paths between an intermediate electrode and two outer electrodes in correspondence with the ratio between chamber voltages, so that, in the case of uniform soiling, no voltage shift occurs at the intermediate electrode.
For the prevention of precipitation on the radioactive source as would impair the operational performance of an ionization smoke detector, published German Patent Document DE-PS-1101370 discloses the inclusion of an electrically biased annular guard electrode facing a conductive support of the radioactive source. The resulting electrical field is intended for preventing precipitates from forming on the radioactive source.
Published German Patent Document DE-B2-2423046 discloses a system of annular guard rings for detecting reduced insulation resistance of the ionization measurement chamber as may result from condensation or dust accumulation. A change in the potential difference between the guard-ring system and the connection between measurement and reference chambers is taken by control and indicating means as a problem indicator.
U.S. Pat. No. 3,964,036, issued Jun. 15, 1976 to Y. Adachi et al. discloses a fire detector system including first and second ionization chambers, one serving as measurement ionization chamber for the detection of smoke, and the other serving as reference chamber. These chambers are connected in series between signal lines which also serve as electrical power supply lines to the detector. An amplifier element is connected to the electrode common to measurement and reference chambers, to produce an amplified signal corresponding to the voltage at the common electrode. The course of the amplified signal of the ionization smoke detector is shown on a display screen and recorded by a plotter. A false alarm is distinguished from a genuine alarm on the basis of a comparison of the shape of the signal so obtained with curves known to be due to soiling or condensation. This method of diagnosing false alarms is expensive, technologically as well as with respect to personnel needed.
To overcome the drawbacks of the described fire detector systems, a method disclosed in published European Patent Document EP-A2-0384209 provides for operation of an ionization smoke detector as follows: The ionization smoke detector includes an ionization measurement chamber in which ambient air is ionized by a radioactive source. Included in the ionization measurement chamber is a first electrode to which a direct-current supply voltage is applied, and a measurement electrode whose electrical potential changes as a function of the concentration of aerosols upon admission of smoke into the measurement chamber. Monitoring of this electrical potential is used for the production of a smoke alarm signal. When, under application of the supply voltage to the first electrode, the electrical potential at the measurement electrode reaches a predetermined first reference value, a second voltage is temporarily applied to the first electrode. The electrical potential at the measurement electrode is now compared with at least one second reference value, and a smoke alarm signal is produced when the electrical potential at the measurement electrode has at least approximately reached the second reference value. The second reference value may be determined based on the law of small-ion adsorption, and on measurements and/or calculations corresponding to smoke-free or essentially smoke-free conditions.
The described detector system provides for immediate and automatic discrimination as to whether a change in the ionic current in the ionization measurement chamber is spurious, or whether it represents a genuine alarm caused by fire. However, this fire detector system does not provide for the detection of smoke when ionization has been reduced by condensation on the radioactive source. More specifically, when the ionic current is reduced simultaneously or previously due to shielding of the radioactive source, e.g., by condensation, the described smoke detector system is unable to detect smoke invasion.